A micro-porous film has been widely used as various battery separators, a separation filter, a micro-filtration membrane, and the like, due to chemical stability and excellent properties thereof. Among them, a separator for a secondary battery has internal pores capable of transferring ions together with a function of spatially blocking a cathode and an anode from each other. Recently, in accordance with high capacity and high output of the secondary battery, as one of the methods for improving electric stability of the battery, a demand for improving the characteristics of the separator has been further increased. In the case of a lithium secondary battery, when thermal stability of the separator is decreased, a risk of overheating, ignition, or explosion of the battery may be present due to a short circuit between electrodes generated together with damage or deformation of the separator caused by a temperature increase due to abnormal behavior of the battery.
Recently, under a condition requiring high output/high capacity of the battery such as an information technology (IT), an electric drive vehicle (EDV), an electric power tool, an energy storage system (ESS), or the like, since ignition possibility and explosion possibility that will be generated at the time of abnormal behavior of the battery may be several times to several ten times higher than those of the existing battery, thermal stability of the separator capable of handling the temperature rise of the battery has been urgently required. The separator having excellent thermal stability means a separator capable of serving to block damage of the separator at a high temperature to thereby block direct short-circuit between electrodes. For example, when a short (short circuit) is generated due to dendrite formed during a charging and discharging process of a battery or foreign materials, heating of the battery may be generated, and in this case, generation of ignition/explosion, or the like, may be suppressed by preventing deformation of the separator.
A polyolefin based micro-porous hybrid film using a high heat-resistance resin has been disclosed in Japanese Patent Laid-Open Publication No. 2002-355938. The high heat-resistance resin is coated on a polyethylene based micro-porous film layer by a phase separation method, but it is difficult to implement efficient permeability by using a resin alone to form pores through phase separation, and a degree of phase separation and uniformity may be significantly changed according to drying conditions such as humidity, temperature, and the like, such that there is limitations in producing a separator having excellent quality uniformity. In addition, there is a problem in that shrinkage of the micro-porous film due to a rapid temperature increase at the time of the abnormal behavior of the battery such as the short circuit may not be effectively blocked. Since heat-resistance of a coating layer is excellent and thermal deformation of the coating layer itself is not generated at 130° C., which is a melting point of the micro-porous film, the shrinkage of the micro-porous film may be partially blocked. However, since resistance is insufficient to completely block the shrinkage of the micro-porous film due to a loose net structure of a polymer resin configuring the coating layer, this method is not suitable for preparing a separator having improved thermal stability.
A method of improving heat resistance of a separator and thermal stability of a battery by introducing a polyvinylidene fluoride copolymer, which is a heat resistance resin, as a coating layer has been disclosed in Korean Patent Laid-Open Publication No. 2007-0080245 and International Patent Laid-Open Publication No. WO2005/049318, but there is a limitation in improving thermal stability of the battery in that the coating layer is easily dissolved or gelated in an electrolyte.
In most of the methods of improving heat resistance, a process of forming a coating layer using a heat resistance resin using an organic solvent is applied. In this case, in order to dissolve the heat resistance resin, a large amount of organic solvent is used. In the case of using the organic solvent, there are disadvantages in that economic efficiency may be deteriorated due to a process of recovering or burning up the solvent after coating and drying and this method is not eco-friendly. In addition, the organic solvent has excellent affinity for the micro-porous film, such that the organic solvent may be absorbed in pores of the micro-porous film during a coating process. Due to the features as described above, in the case of forming a coating layer using a solution in which the heat resistance resin is dissolved, after a drying process, the inside of the pores of the micro-porous film is coated with the heat resistance resin. In the micro-porous film coated with the heat resistance resin, a pore size is decreased, such that permeability may be decreased. In addition, when a shutdown function of the micro-porous film is exhibited at a high temperature, the shutdown function may be hindered by the heat resistance resin coated in the pores. In the case of improving heat resistance using the organic solvent, since there is a factor of inhibiting a basic function of the micro-porous film as well as environmental problems, advantages to be obtained by coating a heat resistance layer may be offset. In addition, even in the case of using the heat resistance resin, swelling and melting may be generated in an organic electrolyte, and particularly, the heat resistance resin is swelled and melted in an electrolyte at a high temperature of 120° C. or more, such that the heat resistance resin strongly tends to be separated from the micro-porous film. Therefore, even though a heat resistance property of the heat resistance resin is excellent, it is difficult to exhibit the heat resistance property.
A method of using water as a solvent at the time of the coating process has been disclosed in Japanese Patent Laid-Open Publication Nos. 2004-227972 and 2005-276503. However, in the case of using this water soluble polymer, since the polymer itself has high affinity for water, there is a disadvantage in that a large amount of water capable of having a negative influence on performance of the battery may remain in the coating layer after drying. The micro-porous hybrid film having a high water content may deteriorate the overall performance of the battery such as cycle and long term storage characteristics of the battery, and the like. Further, in the case of using the water soluble polymer alone, adhesive force between the polyolefin based micro-porous film and the coating layer is not sufficient, such that troubles in a battery assembling process and a problem in stability in the battery may be generated.
A separator for non-aqueous electrolyte secondary battery was prepared using carboxylated methyl cellulose (CMC), which is a water soluble polymer, has been disclosed in Japanese Patent Laid-Open Publication No. 2004-227972, but long term lifetime and cycle characteristics are deteriorated due to high water adsorption of the CMC. In addition, the CMC has a property of being easily broken by deformation applied from the outside at the time of mixing an inorganic material, such that a coating layer may be broken or separated by deformation generated during a battery assembling process, which may have an influence on performance and stability of the battery.
A method of modifying a surface of an inorganic material to secure a water resistance property has been disclosed in International Patent Laid-Open Publication No. WO2008/029922. However, in this method, in view of processes, economic efficiency is decreased since a large amount of energy is consumed in order to volatilize water, which is a solvent used in this method, and additional time and energy are used in order to react a surface modifier treated onto the inorganic material. Further, since a surfactant should be used in order to disperse the modified inorganic material in a water solvent again, the remaining surfactant may have a negative influence on characteristics of the battery.